Hand-held spray guns are conventionally used for receiving two chemically reactive fluids, mixing the fluids internally to the gun, and discharging the fluid mixture onto a work piece. Spray guns are often used in the manufacture of fiber-reinforced resin articles, such as boat hulls, bathtubs, automobile body components, machinery housings and furniture. The spray gun receives a resin stream and a polymerization catalyst or hardener stream, mixes the two streams together in a mixer and discharges the mixed stream concurrent with a spray of chopped glass fibers to deposit a mixture of the resin mixture and fiber on a work surface. The resin system used will vary on the exact application and properties desired. Examples include polyester resin and catalyst systems or epoxy resin and hardener systems.
Typically, polyester resin systems require a significantly higher portion of resin than hardener. Generally the hardener comprises less than 5% of the weight of the mixture, and often between 1/2 and 3%. An exact ratio mix is important to assure proper strength, hardness, and other properties of the cured resin. One type of conventional spray gun utilizes two needle valves corresponding to the resin and catalyst streams that are opened sequentially in an attempt to assure the proper resin/catalyst ratio. Each needle valve includes a plunger having a stem on which a stop may be adjustably positioned. Depression of a trigger on the gun to start the spray results in a lever extension portion of the trigger impacting the stops to open the needle valves. The resin needle valve stop can be positioned closer to the trigger lever extension portion to open the resin needle valve slightly before the catalyst needle valve, starting the flow of the resin through the gun first. This prevents the catalyst from flooding rapidly from the catalyst needle valve into the mixer, in higher than proper proportion to the resin, which would result in depletion of pressure on the catalyst supply line. This initial catalyst-rich mix ratio would be followed by a catalyst-poor mix ratio as the resin begins to flow while the pressure rebuilds in the catalyst line.
When properly adjusted to provide staggered resin and catalyst valve opening, such conventional guns provide improved control over resin/catalyst mix ratios. However, the staggered valve-opening gun described above must be readjusted periodically to maintain the proper valve opening sequence, due to the tendency of the valve stops to shift out of position during use. Additionally, the mechanism described in the above conventional gun utilizes several parts to link the opening of the resin and catalyst valves. This linkage of separate parts is subject to wear and breakage, as well as requiring occasional readjustment.
In contrast to the high polyester resin to catalyst mix ratios described above, other types of resin systems require different ratios. For example, many epoxy resin and hardener systems require a significantly lower ratio of resin to hardener, such as a one-to-one ratio of resin to hardener. While it may not be necessary to stagger the opening of resin and hardener valves in a gun discharging this type of resin system, it is desirable to have resin and hardener valves that are reliably linked for concurrent opening without requiring maintenance and periodic readjustment of a mechanically complex linkage consisting of separate parts.
For all types of resin systems, it is desirable to have as efficient a mixing of the resin and catalyst or hardener as possible to provide the most uniform properties in the cured product.
An additional drawback of conventional guns is the gradual wear of valve seat surfaces and mixer surfaces within the gun over time. Valves worn to the point of leakage or a mixer that loses efficiency due to wear may necessitate replacement of the entire gun at relatively high expense.
Spray guns for fiber-reinforced resin systems often include a glass-strand chopper mounted to the spray gun. Squeezing a spring-biased trigger on the spray gun starts the discharge of a spray of chopped fiber simultaneously with the discharge of the mixed resin and hardener. With some conventional guns, it is possible to depress the trigger only partially, in which case mixed resin and hardener flow but the spray of chopped fibers is not started. The spraying of mixed resin and hardener without fiber is desirable in some applications, such as applying a smooth gel coat or resin coatings to the exterior surface of a resin and glass layup. However, with conventional guns, it is difficult to maintain the trigger in such a position due to resistance offered by the trigger and the inability to define the intermediate position at which resin mixture flows but chopped glass does not.